1. Field of the Invention
The present invention relates to a method and apparatus for feedback control of the air-fuel ratio in an internal combustion engine.
2. Description of the Related Art
Generally, in a feedback control of the air-fuel ratio, a base fuel amount TAUP is calculated in accordance with the detected intake air amount and the detected engine speed, and the base fuel amount TAUP is corrected by an air-fuel ratio correction coefficient FAF which is calculated in accordance with the output signal of an air-fuel ratio sensor (for example, an O.sub.2 sensor) for detecting the concentration of a specific component such as the oxygen component in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with the corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio. According to this feedback control, the center of the controlled air-fuel ratio can be within a very small range of air-fuel ratios around the stoichiometric ratio required for three way reducing and oxidizing catalysts which can remove three pollutants CO, HC, and NO.sub.x simultaneously from the exhaust gas.
In the above-mentioned type of apparatus, however, no consideration is given to long-term changes in the operating characteristics of the engine, for example, changes in characteristics due to deposition of a viscous material such as fine carbon particles originating from lubricant constituents and combustion products at the valve clearance or at an injection nozzle of an electronic fuel injector and changes in characteristics due to such deposition at the rear surface of each cylinder intake valve. In addition, the above-mentioned apparatus has no means for detecting a change of the air-fuel ratio during a transient state such as an acceleration mode or a deceleration mode deviated from the optimum value due to the long-term changes in the operating characteristics of the engine, changes in the gasoline characteristics, or the like. Therefore, if gasoline having low volatility characteristics is used, or if long term changes occur in the engine, the air-fuel ratio becomes lean during an acceleration mode, thereby leading to bad drivability such as non-smooth acceleration. Contrary to this, if gasoline having high volatility characteristics is used, the air-fuel ratio becomes rich during a deceleration mode, thereby increasing the fuel consumption and deteriorating the emission gas characteristics.
Thus, when engine operation characteristics change due to long-term deposits or when low volatility gasoline is used, the air-fuel ratio in an acceleration state becomes relatively lean. Hence, the engine operation deteriorates, e.g., non-smooth acceleration occurs. On the other hand, the air-fuel ratio in a deceleration state becomes relatively rich. Hence, emission and the specific fuel consumption deteriorate. Even when a high volatility gasoline is used, the air-fuel ratio becomes rich in an acceleration state, resulting in the same problems.
A technique for the control of the air-fuel ratio to overcome the above problems has been proposed in U.S. Pat. No. 4,499,882. According to this technique, the air-fuel ratio deviation from a reference air-fuel ratio is detected during an acceleration period of the engine, and the correction amount for transient fuel injection amount correction is calculated in accordance with the detected air-fuel ratio deviation, thereby avoiding the deviation of the air-fuel ratio from the optimum value due to the deposition of viscous material on the rear surface of each cylinder intake valve, and the like, during an acceleration mode, and thus the drivability, the fuel consumption, and the gas emission are improved.
Note that the air-fuel ratio deviation has a relation to the amount of the deposition of viscous material on the rear surface of each cylinder intake valve, which will be later explained.
In the above-mentioned technique, however, in the case of a large amount of deposition of viscous material on the rear surface of each cylinder valve, when a fuel cut-off operation continues for a definite time period, fuel absorbed in the deposition is evaporated and is injected into the combustion chambers. As a result, when a fuel cut-off recovery operation is again carried out, fuel injected from the fuel injection valves is first absorbed by the deposits, thereby reducing the engine speed, thus inviting a rough idling phenomenon, engine stalling, or the like.
Note that fuel cut-off control is effected to stop the injection of fuel during deceleration, thereby improving fuel consumption. The control of the fuel cut-off depends upon the opening of a throttle valve, the engine speed, and the like. For example, when the throttle valve is completely closed and the engine speed is higher than a predetermined fuel cut-off engine speed, fuel cut-off is activated. Contrary to this, when the throttle valve is not completely closed or when the engine speed is lower than a predetermined fuel cut-off recovery engine speed, fuel cut-off is released. In this case, the fuel cut-off engine speed is higher than the fuel cut-off recovery engine speed, thereby obtaining the hysteresis characteristics of the engine speed. In addition, both the fuel cut-off engine speed and the fuel cut-off recovery engine speed are dependent upon engine state parameters such as the coolant temperature of the engine.